(defun my-html-mode-hook ()
  "Switch to `django-html-mumamo-mode' if Django template was opened."
  (save-excursion
    (if (or (search-forward "{%" nil t)
            (search-forward "{{" nil t))
        (django-html-mumamo-mode))))

(add-hook 'nxhtml-mumamo-mode-hook 'my-html-mode-hook)